Understanding How Pollen Causes Hay Fever

Jun 23, 2010Updated 1 month ago

Oak Pollen Can Trigger Pollen Allergy
While food allergiesget a lot of press, pollen allergy – commonly called hay fever – is far more common, thought to affect as much as 10% of Americans. (National Institute of Allergy and Infectious Diseases)

Pollen, the tiny particles by which many plants reproduce, can wreak immunological havoc in sensitive individuals. Pollen seasons and severity can vary widely by plant type, weather and location, and can trigger symptoms ranging from upper respiratory discomfort , or allergic rhinitis, to severe asthma. Symptoms of a pollen allergy can mimic colds, but when they last beyond the usual three days or so, or occur at the same time each year, it might be time to see an allergist.

How Pollen Causes Allergies

Pollen allergy is caused by the chemical make-up of the pollen itself. The two greatest factors in pollen allergenicity are:

  1. Pollen size: Allergy causing pollens are typically 20-40 micrometers in diameter;
  2. Weather: Windy days make allergies worse by dispersing more pollen into the air, while rain weighs pollen down and washes it from the air.
In general, entomophilous, or “insect pollinated” plants cause fewer allergies than anemophilous, or “wind pollinated” plants. Entomophilous plants often have bright flowers, which produce heavy pollens that stick around the flowers for better adherence to bees and other insects. Anemophilous plants, on the other hand, disperse high quantities of fine, powdery pollen capable of windblown travel.

In sensitive individuals, inhaled pollen triggers the release of chemicals called histamines, which dilate small blood vessels in the nose and cause the nasal passages to swell, resulting in the classic allergy symptoms of congestion and respiratory problems. Histamines can also cause itching, irritability, and excess mucus production. Other chemical mediators, like prostaglandins and leukotrienes, contribute additional allergy symptoms.

Rating Pollen Allergy Potential

Thomas Ogren, author of Allergy-Free Gardening, [Ten Speed Press, 2004] has developed the Ogren Plant Allergy Scale, or OPALS, a rating system that can be helpful in identifying the degree to which various plants cause allergies. Plants are ranked on the OPALS scale from 1, the least allergenic, to 10, the most allergenic. Ogren has found that the types of plants least likely to cause allergies are often “showy” flowers with:

  • large brightly colored petals, especially red, orange, blue or pink;
  • male parts deep inside the flower, that contain pollen;
  • polygamous plants with separate female flowers, and specifically female-only plants, which are pollen-free;
  • lightly aromatic flowers, and
  • heavy, sticky pollen that doesn't travel far on wind.
In contrast, plants more likely to cause allergies usually belong to known allergy-causing plant families like the cashew, olive, and spurge families and are often male plants that:

  • produce a white, sticky sap;
  • have long bloom periods (except orchids);
  • have tiny (under 30 microns), light, dry pollen, or produce pollen in great qualities;
  • have strong fragrances;
  • have flowers with exposed stamens, or lack petals or sepals;
  • have light, yellow, off white or greenish flowers, or very tiny flowers; or
  • produce spores.

Pollen Allergy Treatments

The best treatment is avoidance. In the extreme, that means living far from offending pollen producers. Most allergies, however, are treated with either over-the-counter or prescription medications. Some of the more common basic treatments are:

  • antihistamines, which counter the effects of the histamine released by mast cells, and are usually very effective at relieving sneezing and itching. There are two basic types: “non-drowsy” products like Loratadine (Claritin) or similar 24 hour tablets, or Diphenhydramine (Benadryl).
  • decongestants, which act by reducing the swelling and mucous production caused by histamines, and include compounds like ephedrine, phenylpropanolamine hydrochloride, and pseudoephedrine hydrochloride. They can also raise blood pressure, increase heart rate and cause nervousness in some people, and decongestant nasal sprays can create a “rebound” effect that triggers the same nasal passage swelling it is intended to relieve.
  • corticosteroids, which reduce nasal inflammation and inhibit mucus production. Corticosteroid medication is available as a nasal spray in measured-dose spray bottles.
  • cromolyn sodium, essentially a purified salt water, considered safe and effective, although it must be used for up to several weeks for noticeable improvements
Other treatments include combination therapy, using any of the aforementioned medicines, and immunotherapy, or allergy shots, which help reduce the tolerance to a particular pollen through the injection of diluted extracts of the offending allergen. Although allergy shots can be expensive and may cause other allergic reactions, the AAAAI says immunotherapy is successful in up to 90% of patients with seasonal allergic rhinitis and in 70 to 80% of those with perennial allergic rhinitis.

The only way to know what type of pollen might be causing allergic problems is to visit an allergist. In the meantime, avoiding plants that produce windblown pollens can help, as can a variety of over-the-counter and prescription treatments when allergy season descends. Any medical approach to allergies, however, requires consultation with qualified physicians and careful evaluation of the pros and cons of each treatment.


American Academy of Allergy Asthma and Immunology


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